This invention relates in general to improving the crystallographic quality of solid films grown on the surfaces of solid substrates, and more particularly to enhancing epitaxy or preferred orientation to provide relatively large area thin films regularly oriented by means of a practical process.
Much of modern technology makes use of thin solid films on the surfaces of solid substrates. A number of methods have been used to deposit such thin films including thermal evaporation, DC sputtering, rf sputtering, ion beam deposition, chemical vapor deposition, plating, molecular beam deposition and deposition from the liquid phase.
The structure of thin films can be amorphous (that is, the atoms of the film are not arranged in any crystalline order), polycrystalline (that is, the film is composed of many small regions, in each of which the atoms are arranged in a regular crystalline order, but the small regions have no mutual alignment of their crystallographic axes), preferred orientation (that is, the film is composed of many small regions, in each of which the atoms are arranged in a regular crystalline order, and one or more of the crystalline axes of the majority of said small regions are parallel), or epitaxial (that is, the film is predominantly of a single crystallographic orientation). An epitaxial film is a special case of a preferred orientation film in which corresponding crystallographic axes of all the small regions are oriented in the same directions. A thin film can be the same material (that is, the same element or compound) as the substrate, or it can differ in chemical composition from the substrate. If the film is epitaxial, the former is called "homoepitaxy" and the latter "heteroepitaxy."
In general, techniques for obtaining high quality amorphous and polycrystalline films (particularly metals) are well developed and well understood. However, techniques for obtaining high quality epitaxial films and films of preferred orientation are severely limited, and only a limited number of combinations of overlayer film and substrate have been achieved. In most cases, films exhibit a high concentration of crystalline defects [S. T. Picraux, G. T. Thomas, "Correlation of ion channeling and electron microscopy results in the evaluation of heteroepitaxial silicon" J. Appl. Phys. vol 44, pp 594-602 (1973)]. In some cases, high temperatures are required to achieve epitaxy or preferred orientation, and differences in thermal expansion between film and substrate lead to high stresses and sometimes to cracking when samples are cooled to room temperature. Although there are many important technological opportunities for the application of preferred orientation and epitaxial films, particularly in electronic, acoustic, and optical devices, with a few notable exceptions, such films have not been consistently obtained with sufficient quality or in a sufficient number of combinations and orientations to meet the requirements.
Present or conventional methods for obtaining preferred orientation and epitaxial film growth are based on choosing a combination of deposition parameters (such as substrate composition and orientation, deposition method, deposition rate, temperature and pressure) such that the nucleation and growth processes which take place at a microscopic level on the substrate surface favor the growth of the desired film orientation. The fundamental difficulty with this approach is that it is not always possible to control or reproduce all the factors which affect film nucleation and growth. Moreover, this approach limits the number of epitaxial combinations and orientations.
It is an important object of this invention to overcome the shortcomings of conventional methods for producing epitaxial and preferred orientation films and directly influence in a controllable manner the nucleation, growth and orientation of films grown on solid surfaces.
It is another object of this invention to control the crystallographic orientation of thin films grown on solid surfaces in accordance with the preceding object.
It is a further object of this invention to achieve one or more of the preceding objects while reducing the density and magnitude of defects in crystalline thin films grown on solid surfaces.
It is a further object of this invention to achieve one or more of the preceding objects while obtaining epitaxial or preferred orientation films at moderate temperatures and thereby avoid stresses induced by differences in thermal expansion between film and substrate.
This invention results from the discovery that the phenomena of nucleation, growth, and changes in crystallographic orientation that occur during the early stages of film formation on solid surfaces can be influenced and controlled by means of artificial surface relief structures and point defects. It is well known that naturally occurring defects such as steps or point defects on crystal surfaces can act as nucleation sites for deposited material. Some indication of the effects of arrays of point defects on the nucleation and growth of epitaxial films can be found in the work of Distler et al [G. I. Distler, "Epitaxy as a Matrix Replicating Process." Thin Solid Films, vol. 32, pp. 157-162 (1976); G. I. Distler, V. P. Vlasor, V. M. Kaneosky, "Orientational and Long Range Effects in Epitaxy," Thin Solid Films, vol. 33, pp. 287-300 (1976)] who observed that naturally occurring point defects on solid surfaces act as nucleation sites. Distler et al further suggest that the point defects on a surface naturally occur in some form of matrix or lattice and that the orientational effects in epitaxy and crystallization in general are due to the existence of the lattice of point defects.